A switched-mode power converter (also referred to as a “power converter” or “regulator”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. DC-DC power converters convert a dc input voltage into a dc output voltage. Controllers associated with the power converters manage an operation thereof by controlling the conduction periods of switches employed therein. Generally, the controllers are coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”).
Typically, the controller measures an output characteristic (e.g., an output voltage, an output current, or a combination of an output voltage and an output current) of the power converter, and based thereon modifies a duty cycle of the switches of the power converter. The duty cycle is a ratio represented by a conduction period of a switch to a switching period thereof. Thus, if a switch conducts for half of the switching period, the duty cycle for the switch would be 0.5 (or 50%). Additionally, as voltage or current for systems, such as a microprocessor powered by the power converter, dynamically change (e.g., as a computational load on a load microprocessor changes), the controller should be configured to dynamically increase or decrease the duty cycle of the switches therein to maintain an output characteristic such as an output voltage at a desired value.
In an exemplary application, the power converters have the capability to convert an unregulated input voltage, such as 12 volts, supplied by an input voltage source to a lower, regulated, output voltage, such as 2.5 volts, to power a load. To provide the voltage conversion and regulation functions, the power converters include active power switches such as metal-oxide semiconductor field-effect transistors (“MOSFETs”) that are coupled to the voltage source and periodically switch a reactive circuit element such as an inductor or the primary winding of a transformer to the voltage source at a switching frequency that may be on the order of 500 kHz or higher.
A conventional way to regulate an output characteristic of a switched-mode power converter, such as output voltage, is to sense a current in an inductive circuit element such as an output inductor in a forward converter topology or a transformer primary winding in a forward or flyback converter topology, and compare the sensed current with a threshold current level to control a duty cycle of the power converter. The threshold current level is generally set by an error amplifier coupled to a circuit node such as an output terminal of the power converter to regulate the output characteristic. The mechanism to control duty cycle is a signal to turn a power switch “on” or “off.”
A feedback circuit structure wherein a duty cycle of the power converter is controlled by sensing a current in an inductive circuit element and comparing the sensed current to a threshold level that is controlled by an error amplifier is generally referred to as current-mode control. An alternative feedback circuit structure is generally referred to as voltage-mode control wherein a duty cycle of the power converter is controlled by comparing a sawtooth waveform generated by an oscillator to a threshold voltage level controlled by an error amplifier.
In view of the broad application of switched-mode power converters in electronic devices, an area of growing environmental concern is the level of power dissipation produced by a power converter under a no-load or a light-load operating condition. It is generally recognized that power conversion efficiency of a switched-mode power converter at a no-load or a light-load operating condition is substantially lower than its power conversion efficiency at higher load levels.
A design approach to increase power conversion efficiency at a no-load or a light-load operating condition employs an active “burst mode” to control the switching action of a power switch in the power converter. A switched-mode power converter may temporarily disable the switching action of a power switch in a burst mode to reduce switching losses in semiconductor devices and in reactive circuit elements such as inductors and capacitors.
A power converter employing burst mode operation can be designed with a burst mode operational profile based on a feedback voltage. For example, power converter controller ICE3A/ICE3B produced by Infineon Technologies and described in Application Note entitled “ICE3Axxx/ICE3Bxxx CoolSET™ F3 Design Guide,” V1.0, dated August 2004, which is hereby referenced and incorporated herein, describes entering and exiting an active burst mode based on a feedback signal VFB in a control loop. The feedback signal VFB is produced by an error amplifier in the controller that compares an output characteristic of a power converter, such as output voltage, to a reference output voltage value.
The general operation of this controller including entry and exit from an active burst mode can be described as follows: The controller provides an active burst mode function at no-load or light-load conditions to enable the system to achieve low standby power, for example, a standby power less than 100 mW. Active burst mode refers to a controller operating in an active state that can quickly respond to a change in the feedback signal, VFB.
Using a current-mode control scheme, the feedback signal VFB is employed to control power delivery to the output of the power converter. When the output load power is reduced, the voltage of the feedback signal VFB drops. If the feedback signal VFB stays below 1.32 V for a sufficient period of time, the controller enters burst mode operation. The ICE3A/ICE3B controller operates in a burst mode for the feedback signal VFB lying in the range 3.4 V to 4.0 V. When VFB is greater than 4.0 V, the controller switches on, using a current limitation corresponding to 0.25 V across a current-sense resistor in the power converter. When VFB is less than 3.4 V, the controller switches off, and controller current consumption is greatly reduced. The controller quickly leaves burst mode operation and operates normally, i.e., with an uninterrupted duty cycle, when VFB is greater than 4.8 V.
A design issue for power converters employing burst mode operation is to provide uniform entry and exit conditions, i.e., a relatively uniform output power level, for entering the burst mode over a wide range of input voltages to the power converter. For example, a power converter designed to operate from ac mains is frequently designed to operate over a range of ac line input voltages that spans 85 V to 265 V RMS. Under a light-load operating condition, a power converter may not enter a burst mode for low ac line voltages, and the burst mode exiting conditions are thus not uniform over a range of output power. Accurate control of entry and exit from burst mode operation is important to an end customer, for example, due to acoustic noise that can be generated by a power converter operating in a burst mode.
In current-mode control, two feedback loops can usually be identified. In one feedback loop, referred to as the inner current feedback loop, the sensed current is compared with a threshold current level. A second feedback loop, referred to as the outer voltage feedback loop, provides the threshold current level with an error amplifier that senses an output characteristic of the power converter, such as an output voltage. The inner current feedback loop generally becomes unstable in a continuous current mode (“CCM”) of operation when duty cycle increases beyond 50%, regardless of the stability of the outer voltage feedback loop. CCM refers to uninterrupted current flow in an inductor such as an output inductor over a switching period of the power converter. The inner current feedback loop does not become unstable as duty cycle is increased in a discontinuous current mode (“DCM”) of operation. Many power converter designs would suffer serious limitations if duty cycle greater than 50% was not allowed. The stability of the inner current feedback loop is dependent on the slope of the sensed current versus time. By injecting a small amount of slope compensation into the inner current feedback loop, stability of this loop results for all values of duty cycle.
A related design issue for power converters is the number of pins (physical circuit nodes) that is required for power converter control and for interactions with external system elements. For example, a plurality of pins is generally required to manage power converter operation in a burst mode, to sense an input line voltage, to enable an external system to signal shutdown to the power converter, to enable a power converter to signal a delayed restart condition to an external system, etc. The utilization of a plurality of external pins to provide these functions incurs physical space and cost in a power converter design.
Thus, there is a need for a process and related method to provide control of entry into and exit from a burst mode in a switched-mode power converter and to provide control of a power converter with a minimal number of pins that avoids the disadvantages of conventional approaches.